Method for cleaning fine coal

Abstract

A method for cleaning fine coal is provided which includes: mixing the coal with a fluid of such a specific gravity that clean coal particles would float while refuse particles would sink therein, pretreating the coal-fluid slurry by adding a surfactant, subjecting the mixture to ultrasonic dispersion, and separating the entire mixture into higher and lower specific gravity fluid streams by means of centrifugal separation. The fluid of the chosen specific gravity and the surfactant may be recovered from the fluid streams and recycled if desired.

I claim: 
     
       1. A method for cleaning fine coal of refuse comprising: mixing said fine coal composed of a mixture of clean coal particles and refuse particles with a heavy liquid having a specific gravity generally intermediate that of the clean coal particles and the refuse particles to form a slurry; pretreating said slurry, wherein said pretreating includes (a) adding a surfactant as an ingredient in said mixing step such that said slurry includes said surfactant;   (b) subjecting the fine coal heavy liquid surfactant slurry to ultrasonic treatment at a frequency of generally 55,000 cps for dispersing the clean coal particles and the refuse particles; and separating said refuse particles from said fine coal particles by centrifugal separation.     
     
     
       2. A method according to claim 1 wherein said heavy liquid is selected from the group consisting of chlorinated hydrocarbons, liquid chlorofluorocarbon compounds, aqueous solutions of metal salts, and slurries of finely divided high density solids in a carrier liquid. 
     
     
       3. A method according to claim 1 wherein said heavy liquid is selected from the group consisting of chlorinated hydrocarbons and liquid chlorofluorocarbon compounds. 
     
     
       4. A method according to claim 1 wherein said surfactant is an anionic surfactant compatible with coal and soluble in said heavy fluid. 
     
     
       5. A method according to claim 1 wherein said centrifugal separation of said separating step is accomplished by means of a disc centrifuge. 
     
     
       6. A method according to claim 1 wherein said centrifugal separation of said separating step is accomplished by means of a decanter centrifuge. 
     
     
       7. A method according to claim 1 wherein said centrifugal separation of said separating step is accomplished by means of hydrocyclone. 
     
     
       8. A method according to claim 1 wherein a first specific gravity stream containing primarily said clean coal particles, said heavy liquid and said surfactant, and a second specific gravity stream containing primarily said refuse particles, said heavy liquid and said surfactant result from said centrifugal separation, further comprising: recovering said heavy liquid, and said surfactant from said first specific gravity stream; recycling said recovered heavy liquid and surfactant for mixing with the fine coal to product the slurry; and recovering and recycling said second specific gravity stream for mixing with more of said fine coal, said heavy liquid, and said surfactant in said mixing step.   
     
     
       9. A method according to claim 1 for cleaning fine coal of refuse wherein a first specific gravity stream containing primarily said clean coal particles, said heavy liquid and said surfactant, and a second specific gravity stream containing primarily said refuse particles, said heavy liquid and said surfactant result from said centrifugal separation, further comprising: recovering said heavy liquid and said surfactant from said lower and higher specific gravity streams.   
     
     
       10. A method according to claim 9 further comprising: recycling said recovered heavy liquid and surfactant for mixing with the fine coal to produce the slurry. 
     
     
       11. A method according to claim 1 wherein said surfactant is added such that its concentration in said heavy liquid is generally between 200 ppm and 500 ppm. 
     
     
       12. A method according to claim 11 wherein the relative amounts of fine coal to heavy liquid in said slurry is about one liter of heavy liquid to 50 grams of fine coal. 
     
     
       13. A method according to claim 12 wherein the specific gravity of said heavy liquid is in the range of 1.30 to 1.65.